Large capacity Write Once Read Mostly (also known as Write Once Read Many, or WORM) optical storage has recently become available at a reasonable price. New applications have used WORM optical storage to efficiently and economically store vast amounts of data, such as image data, that would normally be archived in warehouses or basements on paper, x-rays, etc. A volume of data is normally stored on one side of a WORM optical disk. A mass optical storage device (also known as a "jukebox", or MOSD) is used to access several WORM optical disks located in an optical library. The MOSD has a mechanical arm or robot that selects one of the WORM optical disks out of the library and moves the disk into a read/write drive.
One optical library can store up to 20,000,000 documents or 2,500,000 images. 128,000 conventional magnetic diskettes would be required to store this massive amount of data. An optical library made up of low cost WORM optical disks could replace a whole warehouse full of paper or other hard copy data. Retrieving archived data located in an optical library is much easier, safer and quicker than looking through literally tons of paper to find the desired document.
For all its benefits, WORM optical storage does have some drawbacks. The biggest drawback is that WORM optical disks can only be written to once. There is no way to erase data that has already been written to a WORM volume. If a document or file stored on an WORM volume needs to be updated, the updated file needs to be stored in addition to the original file. If the volume is almost full and does not have sufficient space to store the updated file, the updated file must be stored on a different volume than the original file.
There are many applications where it is important to have the original file and updated file(s) stored on the same volume. One such application is where the names of image files are kept in one or more indexes in a data base. Storing updated versions of an image on another volume or volumes would require the burdensome task of changing the name of the image file on all volumes where the versions exist.
Another application where it is important to have the original file and updated file(s) stored on the same volume is where an audit trail of all changes to the file is desired. An audit trail requires that both the original file and the updated file be retrieved. Performance of a MOSD is dramatically improved if files that are often accessed together are stored together on a single volume, since retrieving multiple optical disks is slow, inefficient and time consuming.